|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||P8, Fabrication of Subwavelength Pillar Arrays on GaAs by Confined Self-Assembly Technique for Broadband Antireflection Coating
||Dae-Seon Kim, Min-Su Park, Yon-Kil Jeong, Wu Lu, Jae-Hyung Jang
|On-Site Speaker (Planned)
Multilayer thin films are widely used for antireflection coatings of various devices such as solar cells, photodetectors, and optical amplifiers to minimize the optical loss due to surface reflections. Although thin film coatings are ideal for antireflection coatings in narrow band applications, subwavelength structures (SWS) are preferred for broad band applications. So far SWS have been fabricated by nanolithographic techniques such as electron beam lithography, nanoimprint lithography and holographic lithography. Even though these methods have successfully demonstrated SWS with good optical properties, low cost process technologies are highly needed for manufacturing of optical devices requiring broadband antireflection coating.
In this study, SWS on GaAs substrates were fabricated by using the confined convective self-assembly1 method followed by inductively coupled plasma reactive ion etching (ICP-RIE). This method provides a much simpler and cheaper process and opens the possibility of large area SWS fabrication at low cost. Monolayer consisting of 300-nm-diameter polystyrene spheres were prepared on GaAs substrate using the confined convective self-assembly method. Either multilayer or monolayer polystyrene spheres could be assembled on the substrate by controlling the lift-up rate and the substrate temperature. The closely packed monolayer was subsequently treated by O2 reactive ion etching (RIE) process (30 mTorr chamber pressure, 30 SCCM O2). After the exposure to O2 plasma for 60 sec, the diameter of polystyrene spheres decreased from 300 nm down to 250 nm. The resulting 250-nm-diameter sphere arrays with 300-nm-pitch were used as the etching mask for the pattern transfer by ICP-RIE. Subwavelength structures consisting of pillar arrays with aspect ratios of 0.4, 1.35 and 3.63 were fabricated by controlling the etching time of ICP-RIE (7.5 SCCM SiCl4, 60 SCCM Ar). Finally the remnant polystyrene spheres were removed by O2 RIE process. The reflectance of the fabricated SWS was measured at wavelength ranging from 300 to 1000 nm under the normal incidence. Reflectance of the bare GaAs substrate was higher than 30% throughout the wavelength range investigated in this study. For comparison, the GaAs substrates coated with single layer SiNX exhibited very low reflectance of 1% at the wavelength between 550 and 700 nm. The GaAs with SWS in this work exhibited reflectance lower than 8.5% throughout the wavelength range investigated. The low reflectance achieved by GaAs textured by the nano-pillar SWS is attributed to the nature of subwavelength structures which cannot be resolved by the incident light. Continuous transition of the effective refractive index from the top surface to the substrate dramatically reduces the surface reflection by minimizing the refractive index mismatch at the interface2. The fabrication details and the optical measurement results of the SWS with various aspect ratio and various fill factors will also be discussed.